Power semiconductor package with bottom surface protrusions

ABSTRACT

A package includes a body that encapsulates a semiconductor die, the body having a first pair of opposing lateral sides, a second pair of opposing lateral sides, a top, and a bottom. The bottom has a primary surface and a plurality of protrusions that extend outward from the primary surface. When the package is mounted to a printed circuit board (PCB) the protrusions contact the PCB and the primary surface is disposed a first distance away from the PCB. The package further includes a plurality of leads that extend outward from the first pair of opposing lateral sides.

This application is a continuation of application Ser. No.: 12/462,245,filed Jul. 31 2009, entitled, “POWER SEMICONDUCTOR PACKAGE WITH BOTTOMSURFACE PROTRUSIONS” now U.S. Pat. No. 8,207,455, which is assigned tothe assignee of the present application.

TECHNICAL FIELD

This disclosure relates generally to the field of packaging of powersemiconductor devices.

BACKGROUND

Power semiconductor devices which include high-voltage field-effecttransistor (HVFET) devices and other integrated circuits (ICs) thatdissipate high power are well known in the semiconductor arts. Suchpower semiconductor devices are typically encapsulated within a packagethat is designed for assembly onto a printed circuit board (PCB). FIG. 1is a side view of a typical prior art semiconductor package 100 thatincludes a body 110 and a plurality of leads 120 that extend outwardfrom the sides of the body. Each of the leads is wider at the top ascompared to the bottom. Where the width of leads 120 transition from thenarrower bottom part to the wider top part, shoulders 126 are provided.The shoulders 126 of each of leads 120 typically rest on a top surfaceof the PCB (not shown) when the package is assembled. This provides aseparation distance between a bottom surface 140 of package body 110 andthe top surface of the PCB. Although this separation distance is usefulwhen cleaning off the top surface of the PCB, this package designsuffers from a number of drawbacks.

One of the drawbacks with the prior art semiconductor package of FIG. 1,is that the increased width at the top of leads 120 increases theoverall size of the package. This occurs because leads 120 need to beseparated by a minimum distance, referred to as a clearance distance, tomeet certain electrical standards. Furthermore, any downward forceapplied to the top of package body 110 during the initial insertion intothe PCB and/or during attachment of a heat sink is transferred to theshoulders of the leads. This can stress the leads and cause bending,breaking, or disconnecting of the leads (and/or lead wires) connected tothe semiconductor die housed within the package body. In addition, suchdownward forces may also stress the package body causing delamination ofthe internal wiring connected to the semiconductor die within thepackage body.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be understood more fully from the detaileddescription that follows and from the accompanying drawings, whichhowever, should not be taken to limit the disclosed subject matter tothe specific embodiments shown, but are for explanation andunderstanding only. It should also be understood that the elements inthe figures are representational, and are not drawn to scale in theinterest of clarity.

FIG. 1 illustrates a side view of a prior art semiconductor package.

FIG. 2 is a bottom perspective view of an example power semiconductorpackage.

FIG. 3A is a front side view of the power semiconductor package shown inFIG. 2 assembled on a circuit board.

FIG. 3 b is an expanded view of a bottom corner section of the assemblyshown in FIG. 3A

FIG. 4 illustrates a lateral side view of the assembly shown in FIGS. 3A& 3B.

FIG. 5 is a front side view of the assembly of FIGS. 3 & 4 with a heatsink attached to the top of the package.

FIG. 6 is a bottom view of the power semiconductor package shown in FIG.2.

FIG. 7 is a bottom view of another example power semiconductor package.

FIG. 8 illustrates a bottom view of yet another example powersemiconductor package.

FIG. 9 illustrates a bottom view of still another example powersemiconductor package

DESCRIPTION OF EXAMPLE EMBODIMENTS

In the following description specific details are set forth, such asmaterial types, dimensions, structural features, manufacturing steps,etc., in order to provide a thorough understanding of the disclosureherein. However, persons having ordinary skill in the relevant arts willappreciate that these specific details may not be needed to practice theembodiments described. References throughout this description to “oneembodiment”, “an embodiment”, “one example” or “an example” means that aparticular feature, structure or characteristic described in connectionwith the embodiment or example is included in at least one embodiment.The phrases “in one embodiment”, “in an embodiment”, “one example” or“an example” in various places throughout this description are notnecessarily all referring to the same embodiment or example.Furthermore, the particular features, structures or characteristics maybe combined in any suitable combinations and/or sub-combinations in oneor more embodiments or examples.

FIG. 2 is a bottom perspective view of an example power semiconductorpackage 200 housing a semiconductor device or die (not shown). Thesemiconductor die may comprise one of any number of different types ofpower integrated circuits. For example, the semiconductor dieencapsulated within package 200 may include a power output device withan integrated controller circuit for a switching power converter, or ahigh voltage power switch. In another embodiment, package 200 mayinclude a plurality of semiconductor devices or dies.

Package 200 comprises a body 201 made of any one of a number ofdifferent types of packaging materials (e.g., plastic molding compound,ceramic, etc.) that is formed into a rectilinear shape. Body 201encapsulates the semiconductor die and includes opposing lateral sides202 & 204, respective front and back sides 206 & 208, a top 212, and abottom 214. Bottom 214 includes a substantially flat, planar bottomsurface 219 and a plurality of bumps or protrusions 218 that extendoutward from bottom surface 219. In the example of FIG. 2, each of theprotrusions 218 is located near a corresponding corner of bottom surface219.

Package 200 further includes a plurality of metallic leads 216 thatextend outward from opposing lateral sides 202 and 204. In theembodiment shown, six leads are shown extending outward from each ofopposing lateral sides 202 and 204. Other embodiments may have more orless leads 216 extending out of respective lateral sides 202 and 204. Instill other embodiments, additional leads may extend out of front andback sides 206 & 208. In yet another embodiment, leads 216 may extendout of only one side of package 200. Also, an aspect ratio of package200, represented by the length and width of body 201 in the y and xdirections, respectively, may vary in different implementations.

In the example of FIG. 2, opposing lateral sides 202 and 204 each havesubstantially flat surfaces that are substantially parallel to oneanother. Adjacent leads 216 are laterally separated or offset from eachother by a minimum distance (shown as distance d_(CLR) in FIG. 4) tomeet electrical requirements. For instance, the safety standard documentpublished by the International Electrotechnical Commission, IEC 60950-1,provides general guidelines for clearance distance based on the voltageappearing between conductive parts and the environment in which thepackage is used.

Continuing with the example of FIG. 2, protrusions 218 are formedintegral with body 201. For example, body 201 may be formed using aninjection molding process in which substantially non-conductive materialis injected into a mold that encapsulates a power semiconductor die. Amold, therefore, is utilized to form the shape, size, location andpattern of protrusions 218 extending outward from bottom surface 219. Inanother embodiment, protrusions 218 may comprise one or more separatearticles formed of the same similar material as body 201. For example,protrusions 218 may comprise a patterned sheet or matrix of materialthat is adhesively attached to bottom surface 219 during themanufacturing process.

Practitioners in the packaging arts will appreciate that although theexample of FIG. 2 shows four protrusions 218, each located in proximityof corners 217 of bottom surface 219, other embodiments may include anynumber, shape, location and size of protrusions. For example, whereaseach of protrusions 218 are shown in FIG. 2 shaped as a round mound(e.g., like a pitcher's mound) or semi-spherical shape. Otherembodiments may include protrusions 218 each shaped as a cone, aplateau, a pole, an arch cylindrical, block, pyramidal, etc. Moreoverthe size, pattern and location of protrusions 218 may vary in differentimplementations.

FIG. 3A illustrates a front side view of semiconductor package 200 shownafter assembly onto a printed circuit board (PCB) 301. FIG. 3B is a viewof a bottom corner section of the assembly shown in FIG. 3A.Practitioners in the art will appreciate that during the process ofmounting or assembling semiconductor package 200 to PCB 301, leads 216are inserted into through-hole openings 304 of PCB 301 such that acontact area 308 of each protrusion 218 is in contact with an uppersurface 302 of PCB 301. An electrical bonding agent or material 306(e.g., solder) may be utilized to mechanically and/or electricallycouple leads 216 to metal traces or lines running along a bottom surface304 and/or a top surface 302 of PCB 301. In certain installations, asmall amount of epoxy or similar adhesive may be applied to the contactarea 308 of each protrusion to better secure package 200 to PCB 301.

As clearly shown in FIGS. 3A & 3B, when package 200 is mounted to PCB301, contact area 308 of each protrusion 218 contacts top surface 302 ofPCB 301. For purposes of discussion, contact area 308 is defined as theamount of surface area of a protrusion 218 that is in substantiallycontact with surface 302 when package 200 is mounted or assembled ontoPCB 301. In one embodiment, the total area of all contact areas 308 ofprotrusions 218 is about 0.7% or less of the total surface area ofbottom surface 219. In other embodiments, protrusions 218 may be shapedsuch that contact area 308 is substantially zero. For example,protrusions 218 may be designed in the shape of a pyramid or cone whereonly a small tip or point of protrusions 218 contact top surface 302 ofPCB 301.

The expanded view of FIG. 3B further shows how protrusions 218 provide aseparation distance d_(HT) that separates bottom surface 219 of package200 from top surface 302 of PCB 301. Essentially, this separationdistance d_(HT) is equal to the height of each protrusion 218. That is,in the embodiments shown, protrusions 218 all have substantially thesame height d_(HT). In alternative embodiments, protrusions 218 may havevariable heights which allow the separation distance d_(HT) to vary fromone side of the package to another. Such a varying separation distancestill provides sufficient room to clean top surface 302 of PCB 301 witha fluid. In the examples shown in FIGS. 2-5, height d_(HT) isapproximately 250 microns.

FIG. 3B also illustrates one of protrusions 218 being offset or disposedby a distance dl away from an adjacent side edge (e.g., side 202) ofbody 201. In one embodiment, distance dl is approximately 300 microns.However, persons of skill in the art will understand that this distanced1 may vary considerably in different implementations. In certainembodiments, protrusions may be formed coincident with the side surfacesof package 200 (i.e., d1=0).

FIG. 4 illustrates a lateral side view of the assembly of FIGS. 3A & 3B,which shows a set of six leads 216 extending outward from side 202 anddown through PCB 301 in a substantially symmetrical arrangement.Adjacent leads 216 are laterally separated or offset from each other bya clearance distance d_(CLR) to meet electrical requirements. In oneembodiment the clearance distance d_(CLR) may be about 1.27 mm. Whenpackage 200 includes a high voltage device, an adequate clearancedistance d_(CLR) may be required to prevent a short circuit occurringbetween two adjacent leads 216 due to a dielectric breakdown caused bythe ionization of air. As shown, each of leads 216 has a slightlytapered tip or end 220 and an extended length section 221 that has asubstantially constant width d_(LW). Width d_(LW) is substantiallynarrower as compared to the top section of the leads shown in the prior'art design of FIG. 1, which allows the overall length (in they-direction) of package 200 to be considerably reduced. A smallerpackage translates to a lower cost package for a given die size. Inother words, the inclusion of protrusions 218 disposed on bottom surface219 permits the lead width d_(LW) to be reduced, which results in areduction in the overall length L of package 200. In another example, a“creepage” distance, i.e., the distance along the surface of package 200between two leads 216, may be extended with the same package size andsame number of leads 216.

FIG. 5 is a front side view of the assembly of FIGS. 3 & 4 with a heatsink 500 attached to top 212 of package 200. Heat sink 500 typicallycomprises a metallic or other thermally conductive material that isattached to top surface 212 of package 200 using ordinary epoxy 505.Alternatively, various types of other adhesives, a spring clip, a screw,and/or other known adhesive or mechanical techniques may be used tomechanically couple heat sink 500 to top surface 212.

During the process of mounting heat sink 500 to package 200, a large,force (represented by arrows 506) may be applied to secure heat sink 500to top surface 212 of package 200. In another embodiment, force 506 maybe a constant force applied to secure heat sink 500 to top surface 212of package 200. One benefit of the example power semiconductor packagesdescribed herein is that the downward force applied to heat sink 500 issubstantially transferred through body 201 to protrusions 218. Thus,protrusions 218 on the bottom of package 200 helps prevent mechanicalstress from being imparted to leads 216 and solder 306, which aremechanically vulnerable to breakage or to electrically disconnect fromthe semiconductor device housed inside package 200. It is thereforeappreciated that protrusions 218 further improves the robustness andintegrity of package 200 when coupled to heat sink 500.

FIG. 6 is a bottom view of a semiconductor package 600, showing thearrangement of protrusions 618—one near each corner of bottom surface619. As discussed previously, protrusions 618 may be arranged against orcoincident with the edges where bottom surface 619 meets lateral sides602 & 604 and/or respective front and back sides 606 & 608.Practitioners in the art will appreciate that other alternativearrangements or configurations of protrusions 618 may also be employed.For example, FIG. 7 is a bottom view of another example semiconductorpackage 700 in which protrusions 718 are arranged on bottom surface 719such that each of protrusions 718 is disposed at or near the middle ofan edge of bottom surface 719 that adjoins a corresponding one oflateral sides 702 & 704 and front and back sides 706 & 708,respectively.

FIG. 8 illustrates a bottom view of yet another example powersemiconductor package 800 wherein a plurality of protrusions 818 includea plurality of corresponding elliptically-shaped, substantially flatbases 820 arranged on bottom surface 819 in a triangular pattern. Inthis embodiment, two protrusions are located near opposite ends of alateral side 804, with a single protrusion located on bottom surface 819near the middle of lateral side 802. As is the case for any of the otherembodiments described, bases 820 of protrusions 818 may be of anelliptical, triangular, linear, or any other shape.

By way of further example, FIG. 9 shows still another example powersemiconductor package 900 that includes a single protrusion 918 havingan extended linear shape. Protrusion 918 is disposed on bottom surface919 near a front side 906 of package 900. Because this embodimentincludes only a single protrusion 918, bottom surface 919 will lie at anangle or incline in relation to the surface of the PCB after assembly.In other words, when mounted on a PCB protrusion 918 and the edge ofbottom surface 919 that adjoins back side 908 will be in contact withthe PCB. In this manner, a varying separation distance between bottomsurface 919 and the top surface of the PCB is provided.

Although the present, invention has been described in conjunction withspecific embodiments, those of ordinary skill in the art will appreciatethat numerous modifications and alterations are well within the scope ofthe. present invention. Accordingly, the specification and drawings areto be regarded in an illustrative rather than a restrictive sense.

We claim:
 1. A package for housing a power semiconductor devicecomprising: a body that encapsulates the power semiconductor device, thebody having a first pair of opposing lateral sides, front and backsides, a top that provides a mounting surface for a heat sink, and abottom surface, a single elongated protrusion extending outward from thebottom surface such that when the package is mounted to a printedcircuit board (PCB) the single elongated protrusion contacts the PCB andthe bottom surface lies at an angle or incline in relation to a surfaceof the PCB after assembly; and a plurality of leads that extend outwardfrom at least the first pair of opposing lateral sides, each of theleads having a substantially constant width over a length that extendsfrom the body to a point through an insertion hole of the PCB when thepackage is mounted to the PCB.
 2. The package of claim 1 wherein thesingle elongated protrusion is disposed substantially coincident with anedge of the bottom surface.
 3. The package of claim 1 wherein the singleelongated protrusion is substantially linear.
 4. The package of claim 1wherein single elongated protrusion has a substantially flat base. 5.The package of claim 1 wherein the body is rectilinear and the singleelongated protrusion extends substantially across a length of a firstlateral side between first and second corners of the body.
 6. Thepackage of claim 1 wherein each of the leads is separated from anadjacent one of the leads by a distance that is substantially constantover the length.
 7. The package of claim 1 wherein the body comprises amaterial, the single elongated protrusion being integral with thematerial.
 8. The package of claim 1 wherein a force applied to a heatsink during mounting to the top is substantially transferred to thesingle elongated protrusion when the package is mounted to the PCB.
 9. Apackage for housing a power semiconductor die comprising: a body thatencapsulates the power semiconductor device, the body having a firstpair of opposing lateral sides, front and back sides, a top thatprovides a mounting surface for a heat sink, and a bottom surface, aplurality of non-conductive protrusions that extends outward from thebottom surface such that when the package is mounted to a printedcircuit board (PCB) the non-conductive protrusions contact the PCB, atleast one of the non-conductive protrusions having a different height ascompared to a remaining number of the non-conductive protrusions suchthat the bottom surface lies at an angle or incline in relation to asurface of the PCB after assembly; and a plurality of leads that extendoutward from at least the first pair of opposing lateral sides, each ofthe leads having a substantially constant width over a length thatextends from the body to a point through an insertion hole of the PCBwhen the package is mounted to the PCB, wherein each of the leads isseparated from an adjacent one of the leads by a distance that issubstantially constant over the length.
 10. The package of claim 9wherein the non-conductive protrusions are each disposed substantiallycoincident with an edge of the bottom surface.
 11. The package of claim9 wherein the non-conductive protrusions comprise three protrusionsarranged in a triangular pattern on the bottom surface.
 12. The packageof claim 9 wherein the three protrusions each have a substantially flatbase.
 13. The package of claim 9 wherein the body is rectilinear and thenon-conductive protrusions comprise four protrusions.
 14. The package ofclaim 13 wherein the each of the four protrusions is disposedsubstantially adjacent with a corresponding corner of the bottomsurface.
 15. The package of claim 13 wherein the each of the fourprotrusions is disposed at or near the middle of an edge of bottomsurface that adjoins a corresponding one of the opposing lateral sidesand front and back sides, respectively.
 16. The package of claim 9wherein the body comprises a material, the non-conductive protrusionsbeing integral with the material.